Maximizing data rate by adjusting codes and code rates in CDMA system

ABSTRACT

The present invention provides for making code rate adjustments and modulation type adjustments in a pseudonoise (PN) encoded CDMA system. Coding rate adjustments may be made by changing the number of information bits per symbol, or Forward Error Code (FEC) coding rate. A forward error correction (FEC) block size is maintained at a constant amount. Therefore, as the number of information bits per symbol are increased, an integer multiple of bits per epoch is always maintained. The scheme permits for a greater flexibility and selection of effective data rates providing information bit rates ranging from, for example, approximately 50 kilobits per second to over 5 mega bits per second (Mbps) in one preferred embodiment.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of a co-pending U.S.patent application Ser. No. 09/447,022 filed Nov. 22, 1999 entitled“Variable Rate Coding for Forward Link” and a continuation-in-part of aco-pending U.S. patent application Ser. No. 09/263,358 filed on Mar. 5,1999 entitled “Forward Error Correction on Multiplexed CDMA Channels”,each of which are assigned to the same assignee of the presentapplication. The entire teachings of these above referenced applicationsare incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to wireless communicationsystems, and more particularly to a technique for providing variabledata rate connections over digitally encoded radio channels.

BACKGROUND OF THE INVENTION

[0003] The widespread availability of personal computers at low cost hasled to a situation where the public demands access to the Internet andother computer networks at the lowest possible cost. This demand isbeing expanded to include network access for portable devices such aslaptop computers, Personal Digital Assistants, and the like. Users ofsuch portable devices even now expect to be able to access such computernetworks with the same convenience that they have grown accustomed towhen using wire line connections.

[0004] Unfortunately, there is still no widely available satisfactorysolution for providing low cost wireless access to the Internet at highspeed. At the present time, the users of wireless modems that operatewith the existing digital cellular telephone network often experience adifficult time when trying to, for example, view Web pages. The existingdigital cellular telephone network may use schemes such as Code DivisionMultiple Access (CDMA) to permit multiple users to operate on the sameRadio Frequency (RF) physical layer channel at the same time, such as inthe IS-95B standard which is popular in the United States. In thisapproach, each traffic signal is first encoded with a pseudorandom (PN)code sequence at the transmitter. The receivers include equipment toperform a PN decoding function in such a way that signals encoded withdifferent PN code sequences or with different code phases can beseparated from one another. Because PN codes in and of themselves do notprovide perfect separation of the channels, these systems have anadditional layer of coding, referred to as “orthogonal codes.” Theorthogonal codes further reduce interference between channels.

[0005] However, the higher layer communication protocols in suchnetworks were originally designed to support voice communication and notthe packet-oriented data communication protocols in use for theInternet. In addition, the protocols used for connecting users of widearea networks do not lend themselves to efficient transmission overwireless interfaces.

[0006] Certain other CDMA systems have been proposed that moreefficiently support data communications using multiple connections overa given Radio Frequency (RF) channel. One example of such a system wasdescribed in U.S. Pat. No. 6,151,332 entitled “A Protocol Conversion andBandwidth Reduction Technique Providing Multiple nB+D ISDN Basic RateInterface Links Over a Wireless Code Division Multiple AccessCommunication System,” and assigned to Tantivy Communications, Inc., theassignee of the present application. With such techniques, higher speedthroughput can be provided by a more efficient allocation of access tothe CDMA channels. In particular, a number of sub-channels are definedwithin a standard CDMA channel bandwidth, such as by assigning adifferent code to each sub-channel. The instantaneous bandwidth needs ofa given connection are then met by dynamically allocating multiplesub-channels on an as-needed basis for each session. For example,sub-channels can be granted during times when the subscriber bandwidthrequirements are relatively high, such as when downloading Web pages.The sub-channels are then released during times when the content isrelatively light, such as when the user is reading a previouslydownloaded Web page.

[0007] However, to implement such a system requires careful planning ofvarious modulation and coding schemes in order to accomplish the maximumpossible bit rate while minimizing the effects of noise, multi-pathdistortion, and other sources of errors. For example, modulation codesand pseudorandom spreading codes must be carefully selected to minimizeinterference among channels occupying the same radio frequency carrier.In addition, it is necessary for framing bits to be inserted in datastreams so that higher layered data protocols such as TransmissionControl Protocol/Internet Protocol (TCP/IP) can be efficientlyimplemented.

[0008] Furthermore, in order for the PN and orthogonal code propertiesto operate properly at a receiver, certain other design considerationsmust be taken into account. For signals traveling in a reverse linkdirection, that is, from a mobile unit back to a central base station,power levels must be carefully controlled. In particular, the orthogonalproperties of the codes are optimized for the situation where individualsignals arrive at the receiver with approximately the same power level.If they do not, channel interference increases.

[0009] The forward link direction presents a different problem. A signaltraveling from the base station to a subscriber unit may interfere withanother signal in an unpredictable way as a result of the so-callednear-far problem. For example, far away mobile units require relativelyhigh power in order to be detected properly whereas close-in mobileunits require lower power. The stronger signals may interfere withproper operation of mobile units located closer to the base stationwhich typically operate with lower power levels. Unfortunately, thisbehavior depends upon the specific operating environment of the mobilecommunications system, including the topology of the surroundinggeography, the juxtaposition of the subscriber units with respect to oneanother, and other factors.

[0010] In the past, with voice-based systems such as IS-95, it has beenpossible to set power levels individually to optimize each forward linkchannel so that interference is minimized. With these systems, since theinformation bandwidth remains constant, a transmitted power level can becontinuously adjusted in a closed-loop fashion to affect an optimumreceived power level at the subscriber unit which tends to minimizeinterference.

SUMMARY OF THE INVENTION

[0011] Statement of the Problem

[0012] While the above-mentioned systems work well in relativelynoise-free environments, they are not optimal in certain respects.

[0013] Certain techniques known as forward error correction (FEC) aregenerally used with CDMA and other multiple access modulation schemesapplied to voice transmission. Such techniques accept a group of bits,or a “block,” to be sent over a wireless channel and then, according tosophisticated mathematical algorithms, determine values for additionalredundant bits. The number of redundant bits may be quite significant.For example, it is common to use so-called one-half rate, one-thirdrate, or even one-quarter rate codes whereby the number of bits in ablock actually transmitted increases by a factor of two, three, or fourrespectively.

[0014] The forward error correcting code can therefore be used to notonly detect that a particular string of bits has been received in error,but also to implement error correction. This eliminates the need toretransmit an entire packet due to an error in one or more bits. Forwarderror correction has thus been widely used in implementations such assatellite broadcast where retransmission is impractical and/orexpensive.

[0015] Unfortunately, implementation of forward error correction leadsto transmitting fewer information bits per packet. In addition, the needto obtain the best error performance typically dictates that arelatively large block size be used for the highest performancealgorithms. Implementation of such error correction algorithms thereforeincurs latencies in that the entire block must be available at thereceiver before it can be decoded. In addition, if an error is detectedwhich cannot be recovered through the forward error correction process,additional latencies are incurred while the block is retransmitted.

BRIEF DESCRIPTION OF THE INVENTION

[0016] The present invention is a protocol converter disposed between aphysical communication layer, as may be associated with implementing awireless communication protocol, and a network layer, as may beassociated with implementing a network communications protocol.

[0017] In the preferred embodiment, the protocol converter on thetransmitter side first splits a network layer frame, such as a TCP/IPframe, into smaller portions referred to as segments. The segment sizemay be variable in length according to an observed error rate. At thispoint, the segments are then arranged into groups referred to herein asblocks.

[0018] A forward error correction (FEC) algorithm is then applied to theblock as a whole. The rate of the FEC algorithm applied may be selectedfrom a number of available rates, based upon observed channelconditions.

[0019] The block size and FEC code rate are selected to provide apredetermined number of bits per encoded block. For example, the FECcode may be a one-third, a one-half, or a four-fifths rate errorcorrection code for coded blocks of 4096 or 2048 bits. For coded blocksizes of 1024 bits, the code rates may be one-third or two-third ratecodes.

[0020] A symbol modulation process is then applied to the FEC encodedblock. In the preferred embodiment, Quadrature Amplitude Modulation(QAM) is applied with the available symbol encoding rates being selectedfrom 4, 8, 16, or 64 bits per symbol. These symbol encoding ratesimplement Quadrature Phase Shift Keyed (QPSK), 8-Level Phase Shift Keyed(8-PSK), 16-level Quadrature Amplitude Modulation (16QAM) or 64 QAM,respectively.

[0021] The protocol also preferably makes use of multiple physical layerconnections referred to herein as sub-channels to transmit the encodedsymbol blocks at an overall desired transmission rate. Thus, the symbolmodulated block is split among the allocated sub-channels such as on asymbol by symbol basis. The symbols comprising the block are then sentover the sub-channels by further modulating the symbols with apseudonoise (PN) spreading code and a channel code for each sub-channel.The sub-channels are preferably allocated in pairs. This permits thenumber of bits transmitted per PN epoch to remain a power of two, whichsimplifies system design.

[0022] On the receiver side, a receive protocol converter performs theinverse function. Symbols received over the various wirelesssub-channels are first assembled into a received block. The receivedsymbol block is decoded into bits, and then presented to the inverse FECalgorithm to strip off the redundant code bits and perform errorcorrection. The output of the FEC decoding process is then assembledinto the required network layer frames.

[0023] This invention provides an additional degree of freedom to thesystem manager process by permitting individual traffic channel datarates to adapt to specific channel conditions on the fly. For example,an optimum forward error correction (FEC) coding rate may be selected aswell as an optimum modulation type for observed conditions in theindividual channels.

[0024] A fixed number of FEC symbols is thus maintained per transmittedframe, independent of FEC coding rates, power levels, and symbolmodulation type. This allows different FEC rates, symbol rates, and/ordifferent FEC codes to be assigned to each user channel depending uponchannel conditions, without changing the effective transmitted powerlevels.

[0025] For example, if one channel is experiencing relatively goodpropagation conditions, the FEC coding rate and/or the number of symbolsmay be increased per FEC frame without changing transmit power levels.Because the overall information rates depends upon the ratio of the rawdata rate divided by the FEC code rate times the symbol coding rate, ahigher overall information rate is obtained without producing greaterinterference to the operation of other channels.

[0026] On the other hand, if a particular channel is in a relatively bador marginal transmission environment, steps can be taken to reduce theoverall information rate. Specifically, the effective FEC coding ratecan be increased and/or the number of symbols reduced thereby reducingthe effective number of input bits per FEC frame. This permits thechannel to become more robust without increasing the transmit powerlevel.

[0027] In a preferred embodiment, the receiver is notified of the FECcoding rate, symbol modulation rate and other channel parameters byperiodically sending a message to the intended receiver to indicate thecoding rate and symbol rates to be used in future transmissions for agiven channel. In a typical cellular radio communication andimplementation such rate messages may be sent on a forward link pagingchannel, a reverse link access channel, or a synchronization channeldirected to a particular receiver.

[0028] The invention is particularly advantageous in an environmentwhich uses packet-oriented protocols such as TCP/IP. Because the numberof channels needed to carry a single data stream can be variedefficiently, burst rates can also be efficiently adapted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0030]FIG. 1 is a block diagram of a system in which a portable dataprocessing device uses a protocol converter according to the inventionto connect to a wireless network.

[0031]FIG. 2 is a detailed diagram depicting the architecture of theprotocol converter and multi-channel transmitter for a forward link.

[0032]FIG. 3 is a diagram illustrating how network layer frames areencoded into symbol blocks at a transmitter.

[0033]FIG. 4 is a diagram illustrating a protocol converter at areceiver that reassembles the network layer frames.

[0034]FIG. 5 is a chart of information bits rates for differentavailable sub-channels, forward error correction (FEC) rates, and symbolrates given a 4096 block size.

[0035]FIG. 6 is a similar chart for a 2048 block size.

[0036]FIG. 7 is a similar chart for a 1024 block size.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0037] Turning attention now to the drawings more particularly, FIG. 1is a block diagram of a system 10 for providing high speed datacommunication service according to the invention. The system 10 consistsof a remote subscriber unit 20, multiple bidirectional communicationlinks 30, and a central or local service provider unit 40.

[0038] The subscriber unit connects to terminal equipment 12, such as aportable or laptop computer, hand held Personal Digital Assistant (PDA),or the like. The subscriber unit 20 includes a protocol converter 25which provides data to a multi-channel digital transceiver 26 which inturn connects to a subscriber unit antenna 27.

[0039] The protocol converter 25 receives data from the computer 20, andtogether with appropriate hardware and/or software, converts it to aformat suitable for transmission such as in accordance with knowncommunication standards. The protocol converter 25 implements anintermediate protocol layer that coverts the data to a formatappropriate for use by the multi-channel transceiver 26 according to theinvention. As will be described in much grater detail below, at anetwork layer, the data provided by the protocol converter 25 to theterminal equipment 12 is preferably formatted in a manner consistentwith suitable network communication protocols, such as TCP/IP, to permitthe terminal equipment 12 to connect to other computers over networkssuch as the Internet. This description of the protocol converter 25 andprotocols is exemplary only and it should be understood that othernetwork layer protocols can be used.

[0040] The multi-channel digital transceiver 26 provides access to oneor more physical communication links such as the illustrated radiochannels 30. The physical links are preferably further encoded usingknown digital multiplexing techniques such as Code Division MultipleAccess (CDMA) to provide multiple traffic on a given radio channel 30 orsub-channels 31. It should be understood that other wirelesscommunication protocols may also be used to advantage with theinvention.

[0041] The communications channels may be implemented by providingmultiple coded sub-channels 31 on a single wide bandwidth CDMA carrierchannel 30 such as having a 1.25 MegaHertz (MHz) bandwidth. Theindividual channels are then defined by unique CDMA codes.Alternatively, the multiple channels 31 may be provided by singlechannel physical communication media such as provided by other wirelesscommunication protocols. What is important is that the sub-channels 31may be adversely effected by significant bit error rates that are uniqueto each radio channel 30.

[0042] The service provider equipment 40 includes an antenna 42, amulti-channel transceiver 46, a protocol converter 45, and otherinterface equipment 48 such as modems, bridges, gateways, routers, andthe like, which are needed to provide connections to the Internet 49 orother network.

[0043] At the service provider 40, the multi-channel transceiver 46provides functions analogous to the multi-channel transceiver 26 of thesubscriber unit, but in an inverse fashion. The same is true of theprotocol converter 45, that is, it provides inverse functionality to theprotocol converter 25 in the subscriber unit 20. Data is accepted fromthe protocol converter 45 in the TCP/IP frame format and thencommunicated to the Internet 49. It should be understood that theconfiguration of the remaining equipment 40 may take any number of formssuch as a local area networks, multiple dial up connections, T1 carrierconnection equipment, or other high speed communication links to theInternet 49.

[0044] Turning attention now to the protocol converters 25 and 45 moreparticularly, they provide bandwidth management functionality 29implemented between a physical layer such as provided by the CDMAprotocol in use with the multi-channel transceivers 26 and a networklayer protocol such as TCP/IP providing connections between the terminalequipment 22 and the network 49.

[0045] The bandwidth management function 29 performs a number of tasksin order to keep both the physical layer and network layer connectionsproperly maintained over multiple communication links 30. For example,certain physical layer connections may expect to receive a continuousstream of synchronous data bits regardless of whether terminal equipmentat either end actually has data to transmit. Such functions may alsoinclude rate adaption, bonding of multiple channels on the links,spoofing, radio channel setup and teardown. The details for implementinga protocol converter specifically for ISDN terminal equipment 22 andCode Division Multiple Access (CDMA) modulation techniques in use by themulti-channel transceiver 26 are more specifically described in theaforementioned issued U.S. Pat. No. 6,151,332.

[0046] The present invention is more particularly concerned with thetechniques used by the protocol converters 25 and 45 for formatting thedata to be transmitted over multiple logical sub-channels 31-1, 31-2, .. . , 31-n. It should be understood in the following discussion that theconnections discussed herein are bidirectional, and that a “transmitter”may either be the subscriber unit 22 or the service provider unit 40.

[0047] Turning attention now to FIG. 2, there is shown a more detailedblock diagram of a transmitter portion implemented according to theinvention. More particularly, what is illustrated is the transmitter forthe forward link including the protocol converter 45 and multi-channeltransceiver 46 associated with the service provider 40.

[0048] As can be seen from the diagram, the protocol converter 45includes a segmenter 60, block coder 61, Forward Error Correction (FEC)coder 62, and symbol modulator 63. Multi-channel transceiver 46 includesa demultiplexer 64 plus a number of channel modulators including atleast one spreading code modulator 65 and channel code modulator 66. Itshould be understood that there may be a number of spreading codemodulators 65-1, . . . 65-n, and a corresponding number of channel codemodulators 66-1, . . . 66-n, depending upon the number of CDMAsub-channels 31-1, . . . 31-n, being assigned to a particular forwardlink connection. The spreading code modulators 65 preferably apply apseudonoise (PN) spreading code at a desired chipping rate. The channelcode modulators 66 further apply a unique orthogonal or PN code todefine each CDMA sub-channel. In the preferred embodiment, the codingrate is 1.2288 Mega-chips per second with 32 chips per input bit. Asummer 67 adds the various channel signals together. At this point,additional logical channels such as pilot channels and paging channelsmay be added to the data channels before all such channels are fed to aRadio Frequency (RF) up converter 68.

[0049] The controller 69 provides signals that control the operation ofthe segmenter 60, block encoder 61, FEC encoder 62, symbol modulator 63,demultiplexer 64, as well as the allocation of spreading code modulators65 and channel code modulators 66. Specifically, the system may changethe number of bits per block, as applied by the block encoder 61, maychange the particular rate used for error correction coding as appliedby FEC block 62, may change the specific number of bits per symbolimplemented by the symbol modulator 63, and may change the number ofspreading code modulators 65 and channel code modulators 66 allocated toa particular connection. It is the flexibility in assigning thesevarious parameters that provides for a number of degrees of freedom indetermining the forward link capacity for specific connections.

[0050] The overall information rate can be represented by the expressionshown in FIG. 2. This is the ratio of the chip rate divided by thenumber of chips per symbol times the number of bits per symbol used inthe symbol modulator 63, number of code words per connection asimplemented by the number of channel codes implemented by the channelcoders 66, and the ratio of the information block size divided by theFEC block size as implemented by the block encoder 61 and FEC encoder62.

[0051] Continuing now to refer to FIG. 2 in connection with the diagramof FIG. 3, input data is first received such as in the form of ahigh-level network layer frame. Specifically, the input network layerframe 80 may be a group of 1480 data bits in the format of aTransmission Control Protocol/Internet Protocol (TCP/IP) frame. Theframe segmenter 60 reformats the TCP/IP frame, dividing it in thepreferred embodiment into a number of individual segments 81. The sizeof the individual segments 81 is chosen based upon an optimum segmentlength determined for each of the radio channels 30. For example, abandwidth management function 29 may only make available a certainnumber of sub-channels 31 to each network layer connection at a giventime. The optimum number of bits per each segment intended to betransmitted over the respective sub-channels is then chosen. Parameterssuch as the frame overhead, shared frame segmentization flags betweenframes and sub-frame error ratio may be used in determining the segmentsize. For more information on the selection of a particular size for agiven segment 81, reference may had to the above-referenced co-pendingapplication Ser. No. 09/263,358 filed on Mar. 5, 1999, entitled “ForwardError Correction on Multiplexed CDMA Channels”.

[0052] After the input frame 80 is divided into segments 81 by thesegmenter 60, each of the segments 81 typically has additionalinformation appended to it. For example, each of the segments 81 mayhave a position identifier 82A and an integrity check sum such as in theform of a Cyclic Redundancy Check (CRC) 82B. The position identifier 82Aserves to indicate the position of each segment within its associatedlarger frame 80. Because the data bits are ultimately going to bepotentially split and sent among a number of different radiocommunication channels, the integrity check serves to permit thereceiver to determine whether each particular segment has been receivedcorrectly or an error and then subsequently request retransmission ofonly the segment 81 received in error rather than the entire TCP/IPframe 80.

[0053] In any event, regardless of whether or not or how thesegmentation process takes place the bits are then further prepared fortransmission over each sub-channel 31.

[0054] In a next step, the segments 81 are fed to the block encoder 61.The block encoder 61 groups the bits into a predetermined block size.The block size depends upon ultimately the desired number of bits perForward Error Correction (FEC) block output by FEC encoder 62. Inparticular, in the example being described the number of bits desired tobe output by the FEC encoder 62, each block is 4096. In the examplebeing described, the FEC algorithm being implemented is a one-half rateencoder. Thus, the block encoder 61 will first output a group of 2048bits.

[0055] Next, the desired FEC algorithm is applied to the block by FECencoder 62. The FEC encoder applies the desired algorithm outputting theFEC encoded block 84. In the example being described, this FEC encodedblock consists of 4096 bits. In a case where a one-third rate FEC codeis chosen, the block encoder selects 1331 bits; however, the FEC encodedblock will still be 4096 bits long.

[0056] The FEC encoded block is then fed to the symbol modulator 63. Thesymbol modulator 63 groups the bits according to a number of bits persymbol. In the illustrated embodiment, symbol encoding is 4 bits persymbol, i.e., the modulation type selected is Quadrature Phase ShiftKeyed (QPSK). Thus, the symbol encoded block 85 consists of 1024symbols, each symbol having one of four different values that specify aphase.

[0057] Finally, the symbols are then allocated among a number of codechannels. In the illustrated embodiment, the number of code channelsassigned to the particular connection is n. The demultiplexer 64 thusdivides the stream of symbols from the modulator 63 into n separatesymbol streams, each of which is applied to one of the code channels. Itshould be understood that the order of the symbol modulator 63 anddemultiplexer 64 may be reversed; e.g., the demultiplexer 64 may operateon the FEC coder 62 output, and such output may be fed to n symbolmodulator 63. Each respective one of the code channels then has appliedto it its assigned spreading code 64-1 and channel code 65-1, aspreviously described.

[0058] A bandwidth management function associated with the centrallylocated base station equipment 40 determines how many channels to beallocated to each connection. In the case of the present invention, thisbandwidth management function 29 also sets the values for the blocksize, FEC code rate and symbol rate information needed, respectively, bythe block encoder 61, FEC encoder 62, and symbol encoder 63. Thisinformation may be further fed from the bandwidth management function 29down to a controller 68 which distributes such information to theseblocks. A similar controller 90 in the receiver also obtains informationconcerning the specific number of channels, n, symbol rate, FEC codingrate, and block size associated with each connection. Such informationmay be provided by the bandwidth management function 29 in response toobserved conditions in the assigned channels. These adjustments may bemade, for example, in response to determining a signal strength valuewhich may be done by measuring a ratio of the energy per data bitdivided by a normalized noise power level (Eb/No) at the receiver. Thereceiver can therefore periodically measure such normalized noise powerlevel and make a report of such level back to the central base station40.

[0059] For example, if a remote access unit 20 is located deep inside ofa building it may be experiencing particularly adverse multi-path orother distortion conditions. In the past it was thought necessary toincrease the power level of the individual signals 31 in order to obtainan appropriate receive signal level from the access unit 20. However,with the invention, if a full maximum data rate is not needed, then theFEC coding rate implemented by the FEC encoder 62 can be increasedand/or the symbol rate implemented by the symbol encoder 63 can belowered, either or both will result in improved performance.

[0060] In other environments, where multi-path distortion is minimal,such as in a direct line of sight situation, the highest rate for thesymbol encoder 63 may be selected. In addition the highest FEC rate,i.e., the most number of data bits per FEC encoded symbol may beselected by the FEC encoder 62. These can furthermore be selected whileat the same time reducing the radiated power level on the forward linkfor that particular channel. This therefore maximizes the available datarate for a given user while also minimizing the interference generatedto other users of the same radio channel.

[0061] Thus, in environments where radio propagation is good, the systemcan then increase the data rate to a given user on the forward linkwithout introducing additional interference to other users. However, ina bad signaling environment, an advantage is also obtained since eachparticular user channel can be made robust without increasing its powerlevel.

[0062] Turning attention now to the discussion of the receiver in FIG.4, a controller 90 executes a process which sets various parameters ofthe components of the multi-channel transceiver 26 and protocolconverter 25. These include the needed information concerning symbolrate for the symbol demodulator 91, the FEC coding rate by the FECdecoder 92, the block size needed by the block decoder 93, and segmentinformation needed by the segment disassembler 94.

[0063] In the multi-channel receiver 26, an RF down conversion circuit71 provides a number of RF channels. A number, n, of receiver circuitsindividually process these signals to regenerate the sub-channelsignals. In particular, a despreader 73 and channel separator 74,operate to reconstruct the individual sub-channels 31 at the receiver.The despreader 73 removes the PN spreading code applied at thetransmitter by the spreader 64. The channels separation block 74 removesthe channel code applied by the channel coder 65. The resulting nsub-channel signals are then remultiplexed by the multiplexer 75 toproduce a base-band signal consisting of a symbol stream. Thesebase-band symbols are then combined and forwarded to the symboldemodulator 91. In the illustrated embodiment, being discussed inconnection with FIG. 3, the symbol demodulator 91 is a QPSK typedetector. A block assembler 92 groups the demodulated symbols accordingto the FEC block size in effect.

[0064] Next, in connection with the protocol converter 25, an inverseFEC decoding process is applied by FEC decoder 93. The FEC decoded bitsare then provided to the segment disassembler 94. The segmentdisassembler 94 then outputs TCP/IP formatted frames that may be used bythe terminal equipment 12.

[0065] As has been described above, being able to change the symbolmodulation rate, FEC encoding rate, and block size provides severalincreased degrees of freedom in choosing the overall available datarate. FIG. 5 is a chart associated with available data rates assuming aconstant block size of 4096 bits and a fundamental minimum data rate of49.9125 kilobits per second (kbps), or approximately 0.50 Megabits persecond (Mbps). In particular, the chart shows on the various rowsthereof an assumed number of channel codes assigned to each connection.These range from 2, 4, 6, or 8 channel codes, up to a maximum of 28codes being assigned per connection. The columns represent differentcombinations of symbol modulation rate implemented by the symbolmodulator 63, FEC coding rate implemented by the FEC coder 62, and blocksize implemented by block encoder 61. Specifically, the right-mostcolumn indicates a situation where there are four symbols per bit, i.e.,QPSK modulation is selected for the symbol modulator 63. As indicated bythe first number in the table heading, a block size of 4096 has beenselected along with a FEC coding rate of one-third or 1331 bits per FECblock. The cumulative effective data rates that is therefore availablein the case of assigning only 2 channels is 0.50 MHz.

[0066] It is seen that as the number of assigned channel codesincreases, the overall data rate achievable may be increased up to 0.699megabits per second, which is the last column entry. Faster data ratesare available by, of course, decreasing the effective FEC coding rate.For example, in the case represented by the second column from theright, a one-half rate code is selected, or 2048 information bits perblock of 4096 bits (this is the example that was described in connectionpreviously with FIG. 3). In this instance, it is seen that the overalldata rates have been increased to provide a range of 0.076 Mbps up toand including 1.065 Mbps.

[0067] An increase in the FEC coding rate to a four-fifths rate codesuch that there are 3249 information bits per block size of 4096provides even further increase in information bit rate, ranging from0.122 Mbps up to 1.706 Mbps.

[0068] Faster data rates are available also by providing a change in themodulation type, i.e., number of bits per symbol. In the caseillustrated the system supports 8, 16, or 64 bits per symbol effectivelyrepresenting 8-PSK, 16 QAM, or 64 QAM symbol modulation. The availablerates increase as indicated in the table.

[0069] In the maximum data rate case, 28 sub-channels have been assignedto a given connection with a modulation rate of 64 bits per symbol and aFEC coding rate of four-fifths. This combination, which is representedby the last entry in the left most column of the table, provides aninformation rate of 5.117 Mbps.

[0070] A similar table is illustrated in FIG. 6 for the case where theblock size has been reduced to 2048.

[0071] Finally, FIG. 7 is a table for a 1024 block size. In thisinstance, only FEC code rates of one-third or two-thirds make sense, inthat no four-fifths rate equivalent code is possible (i.e., 1024 times ⅘is not a whole number). However, there is still a wide range of datarates available, from approximately the 50 kilobits per second, up to arate which is the neighborhood of 4 Mbps.

[0072] By making code rate and symbol modulation rate adjustments inconnection with a given block size, the PN codes used for thedispreading function and channel codes may be known to roll or shift intime in a known rate with respect to each symbol. For example, given achannel code rate of 32768 chips, rolling over 1024 symbols at 32 chipsper symbol, higher symbol rate type modulation schemes that employ 3, 4,or 6 information bits per symbol (corresponding to the 8, 16, and 64modulation type shown in the tables) are still compatible. In this way,no matter which modulation or symbol rate scheme is selected, there isalways an integer number multiple of 1024 bits per epoch.

[0073] Assigning always at least 2 codes per user ensures that there areeven number per blocks per PN epoch. This provides for increasedsimplicity in the implementation of the receiver, i.e., if there were anodd number of blocks per epoch, it would be necessary to buffer a blockfor a following epoch before being able to complete the construction ofa frame.

[0074] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method for coding channels in a wirelesscommunication system in which a digital signal is communicated from atransmitting station to a receiving station, the method comprising thesteps of: grouping the bits of the input signal into blocks, a size ofeach block being adjustable according to a predetermined block sizeparameter; forward error correction (FEC) coding the bits of the blocks,a rate of the FEC code selected such that a number of FEC symbols in atransmitted block remains constant, even if a number of information bitsin a block changes; symbol modulating the FEC symbols of the blocks witha predetermined number of bits per symbol, again such that the number ofmodulated symbols in a transmitted block remains constant; channelcoding the modulated symbols with a spreading code and a channel code toproduce a transmit signal; and transmitting the transmit signal over awireless communication link.
 2. A method as in claim 1 wherein thenumber of encoded symbols in each transmitted frame remains the same,even if a symbol encoding rate is changed for a given connection.
 3. Amethod as in claim 1 wherein a symbol modulator rate is selected from agroup consisting of Quadrature Phase Shift Keyed (QPSK), eight levelPhase Shift Key (PSK), sixteen level Quadrature Amplitude Modulation (16QAM) and 64 QAM.
 4. A method as in claim 1 wherein the number of FECsymbols per modulator symbol is selected from the group consisting of 2,3, 4, and 6 bits per symbol.
 5. A method as in claim 1 additionallycomprising the step of: sending a message to the receiver station fromthe transmitter station, the message including an indication of thecoding rate used in generating the encoded frames, thereby permittingthe receiver station to determine a symbol decoding rate required toproperly decode the symbols of the received frame.
 6. A method as inclaim 1 additionally comprising the step of: coding each encoded symbolwith a channel code to permit separation of the encoded symbols fromother encoded symbols transmitted on a given radio carrier frequencysignal intended for other channels.
 7. A method as in claim 1 whereinthe communication link is a forward link transmitted from a base stationtransmitter in a direction towards an access unit receiver station.
 8. Amethod as in claim 1 wherein the communication link is a reverse linkchannel transmitting information from a remote subscriber unit stationedtowards a receiving base station.
 9. A method as in claim 1 wherein thesymbol encoding rate is chosen based upon observed link qualityconditions in the radio channel.
 10. A method as in claim 9 in whichradio channels experiencing bit error rates cause selection of a symbolcoding rate which is lower.
 11. A method as in claim 1 wherein symbolencoding rates for different receivers on a given radio carrierfrequency have different symbol and framing rates.